1. Field of the Invention
The present invention relates to electronics, and, in particular, to PIN-diode attenuators.
2. Description of the Related Art
FIG. 1 shows a schematic diagram of a conventional PIN-diode attenuator 100 used, for example, to adjust signal levels, control impedance mismatch, and/or isolate circuit stages in electronics. PIN-diode attenuator 100 is a voltage-controlled attenuator that receives an input signal at input node IN and presents an attenuated output signal at output node OUT, where the level of attenuation is a function of the (positive) bias voltage BIAS applied at the two bias nodes 112 and 114.
Attenuator 100 comprises 90-degree hybrid 102, DC-blocking capacitors C1 and C2, bias resistors R1 and R2, and PIN diodes D1 and D2. Hybrid 102 splits power applied at its input port 104 evenly between its direct port 106 and its coupled port 110, with a 90-degree phase offset between the two output signals. Hybrid 102 also includes isolated port 108. Bias resistors R1 and R2 convert the bias voltage BIAS applied at bias nodes 112 and 114 from a bias-voltage source (not shown) into currents for driving the current-controlled PIN diodes D1 and D2.
By matching and applying equal currents through PIN diodes D1 and D2, the reflection coefficients at coupled port 110 and direct port 106 will be equal, thereby providing a good (i.e., low) voltage standing wave ratio (VSWR) (i.e., the ratio of the reflected power to the forward power), which implies better impedance matching at the input and output ports IN and OUT. The presence of 90-degree hybrid 102 makes attenuator 100 a non-reflective attenuator that maintains a relatively stable return loss over different attenuation levels as opposed to reflective attenuators that provide a good VSWR at only one level of attenuation.
FIG. 2 shows a graphical representation of the relationship of the level of attenuation (in dB) provided by PIN-diode attenuator 100 of FIG. 1 as a function of the control voltage BIAS (in volts) applied at bias nodes 112 and 114 for three different operating temperatures (−40 C, 23 C, and 85 C). FIG. 2 is based on an implementation of PIN-diode attenuator 100 of FIG. 1 in which:                90-degree hybrid 102 is a 900-MHz GaAs monolithic microwave integrated circuit (MMIC) hybrid;        Capacitors C1 and C2 are 10 pF capacitors;        Resistors R1 and R2 are 18 kohm resistors; and        Diodes D1 and D2 are SMP1304 series PIN diodes available from Skyworks Solutions, Inc., of Woburn, Mass.As shown in FIG. 2, not only does the operation of attenuator 100 vary as a function of temperature (with attenuation generally decreasing as temperature increases), but attenuator 100 exhibits undesirable non-monotonic behavior at control voltage levels greater than about 11 volts.        